Historically, reactors were designed in single reactor modules having separate furnaces for each reactor, but the ever-rising price of energy has caused much interest in integrated reactor systems that maximize energy savings.
One such process is a lube oil hydrofinisher process involving a dewaxing reactor and a finisher reactor. These two reactors are connected in series, with only one furnace. In this process, a feedstock is heated in the furnace, then the heated feedstock is dewaxed in a dewaxing reactor, the effluent from the dewaxing reactor is cooled in a first heat exchanger then further cooled by adding a quench, then the cooled effluent is finished in a finisher reactor, and the effluent from the finisher reactor is cooled in a second heat exchanger. Part of the pre-furnace feedstock is used as coolant for the two heat exchangers, the amount of coolant being controlled by a first bypass valve for a line bypassing both heat exchangers and by a second bypass valve for a line bypassing only the second heat exchanger.
While such a process, by being integrated, has major energy cost savings it is more difficult to control. Because of its integrated nature, moving one valve has an effect on both reactors. Since each valve has a multiple effect on the system, there is more than one way of controlling temperatures in such a process. But there is only one combination of positions that maximizes energy savings. For instance, in this process it is desirable to maximize the pre-furnace feedstock temperature so as to reduce the amount of energy that has to enter the system through the furnace. Therefore, it is desirable to have a control process that controls the temperatures of the system in such a way to maximize the pre-furnace feedstock temperature while maintaining stable control.